Incorrect null check compilation error

TypeScript Version: 3.1.0-dev.201xxxxx

Search Terms:
null check, null check compilation

Code

declare const x: (() => void)|null;
declare const ctor: Function;

if (x instanceof ctor) {
  x();
}

Expected behavior:
No compilation error since if x == null => true, then x instanceof Function => false

Actual behavior:
Cannot invoke an object which is possibly 'null'.
const x: (() => void) | null

Playground Link: http://www.typescriptlang.org/play/#src=declare%20const%20x%3A%20%7By%3A%20()%20%3D%3E%20%27hi%27%7Cnull%7D%3B%0D%0Adeclare%20const%20ctor%3A%20Function%3B%0D%0A%0D%0Aif%20(x.y%20instanceof%20ctor)%20%7B%0D%0A%20%20x.y()%3B%0D%0A%7D

Related Issues:

(Note that strictNullChecks must be enabled in Options)

The following code has no compile errors:

declare const x: (() => void) | null
declare const y: FunctionConstructor

if (x instanceof y) {
	x()
}

VSCode editor also points:

But i don't know really what any of that means

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